Phenomena of Inheritance 105 



Modifying Factors. Morgan and Bridges have found that the 

 effects of many factors may be modified by other factors. Thus 

 the eye color of Drosophila known as "eosin" may be modified by 

 six or seven different factors, occupying different loci in the 

 chromosomes, one of which intensifies "eosin" while the others 

 dilute it. These modifying factors are undoubtedly like other 

 Mendelian factors in their behavior and they show that an adult 

 character may be the result of several different inheritance fac- 

 tors. Indeed Morgan says "that an overstatement that each fac- 

 tor may affect the entire body is less likely to do harm than to 

 state that each factor affects only a particular character." And 

 again he says, "It cannot too insistently be urged that when we 

 say a character is the product of a particular factor we mean no 

 more than that it is the most conspicuous effect of the factor" 

 (Morgan, 1916, p. 117). 



Lethal Factors. Morgan and his associates have also demon- 

 strated the existence of a considerable number of lethal factors 

 in Drosophila that cause the early death of those gametes or 

 zygotes in which this factor is not balanced by a normal one. 

 This phenomenon greatly modifies expected Mendelian ratios 

 for only heterozygotes survive, and all individuals that are homo- 

 zygous for a lethal factor usually die so early that they are never 

 seen. Nevertheless their existence can be determined by indirect 

 methods that will be mentioned in the next chapter under "link- 

 age." Such lethal factors greatly complicate the study of genetic? 

 but they do not destroy its fundamental principles. 



What are factors? Inheritance factors are probably complex 

 chemical substances which preserve their individuality in various 

 combinations, just as groups of atoms or radicals do in chemical 

 reactions ; they may be dropped out or added, substituted or trans- 

 posed, just as chemical radicals may be in chemical compounds. 

 To this extent they maintain continuity and independence, but they 

 are not absolutely independent for they react upon one another 

 as well as to environmental changes, so that the characters of 



